In tooth flank grinding machines that work according to the hobbing method, the grinding wheel, for instance a worm gear and the work-piece must run at a specific rotational relationship to each other with extreme precision. The same applies to a hobbing machine.
A method for the establishment of a control value is known as set forth in German Pat. No. 890 420, in which for each of two or more movements that are to be brought into dependence of each other, a pulse sequence is developed. The pulse sequences are compared to each other in respect of their frequencies. On the basis of the value and direction of the control value produced, one of the movements can be controlled until an argreement of the pulse sequences is attained.
Higher precision can be attained if, according to German Pat. No. 890 420, deviations from the phase position of two pulse sequences, derived from the movement, are used for the establishment of the control values. Here, the control value serves the readjustment of one of the movements, so that the movements can be brought into any desired dependence of each other, by using various divisions in respect of the moving bodies, e.g. the tool or the work-piece and by utilizing electrical pulse division or pulse multiplication.
A machine tool that is based upon the comparison of phase position of two speed-dependently produced pulse series, for the production of gears according to the hobbing principle, is disclosed in German Patent application 24 44 975. In this machine tool, the angular encoders that are assigned to the tool and the work-piece, produce an pulse number per revolution that is either equal to or deviates from each other by an even-numbered multiple. A pulse comparator only compares the pulse of the tool angular encoder that corresponds to the impulse of the desired number of teeth of the work-piece vis-a-vis the position of the rise of its slope with the rise of the slope that is given by the pulse of the work-piece angular encoder. This causes a synchronization of the drives in their dependence on the pulse slope-rises, since the control value that is produced by the impulse comparator is proportional to the angular difference of the two drives.
This known comparison of the phase position of the two pulse sequences that were produced in dependence on the rotation (rpm), shows the disadvantage that the measurement of the lead and lag that was taken by the pulse comparator is dependent upon the speed of rotation of the work piece since a time-dependent phase analysis takes place. If the angular speed of the work piece is not constant, then the phase signal is wrong in this technique. The prior art machine took therefore, has insufficient accuracy for the production of precision gears.
It is the task of the present invention to produce a method of the type mentioned above, in which the phase comparison is independent of the speed of rotation of the tool. In order to solve this task, the invention uses the resolution of the produced regulating value by impulses of the tool drive, in multiples several times more numerous than by time unit.
According to the invention, the method of the type mentioned above is characterized in that the difference between the number of pulses of the tool drive, divided by the number of teeth of the work piece, and the number of pulses of the work piece drive is developed continuously, whereby the number of pulses of the tool drive per time unit is greater by a multiple than that of the work piece drive in the same time unit. The results of the formation of the difference and impulse count, are transformed, independently, into analog signals which are summed for the formation of the control signal, whereby the conversion of the result of the impulse counting takes place in dependence on the reciprocal of the number of teeth of the work-piece.
As a result in the method of the present invention, an analog control signal is formed not only from the difference of the count of the work piece pulses and the tool pulses divided by the number of teeth, but also the tool pulses are counted in the time interval between two work piece pulses and, under consideration of the number of teeth, are transformed into an additional analog signal. Hence, a grid of pulses may be interjected between two work-piece pulses permitting a spacially precise phase comparison that is independent of the rotation rate of the tool, since the pulses of the tool correspond to a turning angle, rather than to a temporal displacement and, therefore, are independent of time and speed.
The arrangement according to the present invention is characterized by the following: an adjustable dividing counter is connected to the angular encoder of the tool drive, which, upon reaching the set counting position delivers a pulse signal at a first output and resets itself automatically, and which delivers continually, at a second output, the current counting position; the first output of the dividing counter and the angular encoder of the work-piece drive are each connected to an input of a balance counter; the second output of the dividing counter and the output of the balance counter are connected by a respective gate circuit controlled by the pulses of the angular encoder of the work-piece drive, to a respective storage unit; and a digital-analog converter is connected to each of the storage units, whose outputs are additively connected with each other for the formation of the regulating signal, whereby a constant reference current to be supplied to the digital-analog converter is assigned to the balance counter, while a proportional reference current is supplied to the digital-analog converter is assigned to the divider counter. This reference current is proportional to the reciprocal of the number of teeth of the work piece.
The arrangement of this invention permits the accomplishment of the phase comparison and, hence, a control signal for the adjustment of the work piece drive, without a large number of electronic switching devices, such as counters, storage units, digital-analog converters. The resolution is determined by the pulses provided by the angular encoder of the tool drive and is independent of the speed of the rotation of the grinding wheel drive.
The process, according to this invention will be further clarified in the following description of the preferred embodiment.